Like the subject says...why should you change the cylinder volume when going from a 350 to a 383?

I was out doing some tuning yesterday and got everthing fairly close. Then I noticed that somehow my cylinder volume got changed back to a 350 i/o a 383. I know I changed this before. This sucks.
As soon as I changed the volume back to 383 she started running rough again. I get to start over again.

 

On SD systems the ECM needs to know the engine displacement. This value is used when calculating the mass of air that enters the cylinder. From that value (along with AFR) the ECM can then use the injector constant to calculate the required mass of fuel.

By using the cylinder volume of a 350 on a 383 the VE tables had to make up the difference.

RBob.

 

Hmm. Thanks for the info.
Apparently I need to re-visit my spark tables as well.
This blows. Especially with thirdgen fest coming up this weekend.

 

Yes, you can tune the main VE tables and the P/E without adjusting the cylinder volume and have the engine run quite nicely - especially when the volume change isn't that great (like 350 to 383). But, you may find a lot of "other" tuning areas are affected (such as "pump shot") that also rely on this info somewhere in there calculatiosn and this results in poor drivability.

If you don't adjust the cylinder volume, you may end up having to do a lot of extra tuning to resolve those driveabilty issues. Whereas, if you change the cylinder volume, those issues may not even arise.

I say this, because in the past on "big displacement engines with large injectors", we had to trick the ECM into thinking it was a smaller engine with smaller injectors because there were some calcuation errors in the eprom (until they were fixed). And, once that error was fixed and the proper cylinder volume and injector size could be used, MANY of those driveabilty issues disappeared.

 

Will a cylinder volume of a 350 in a 383 cause it to run leaner than it would if the cylinder volume was for a 383 in a 383? I ask this cause I noticed that I had to add a lot of fuel in pretty much every cell starting at 1400rpm.

Grim - that may explain why i'm having a bear of a time trying to tune the 383 with a cylinder volume of a 383. My idle surge is back with the correct cyl volume. This was with the incorrect cylinder volume value.

On an interesting side note, I was able to eliminate the idle surge by knocking the closed throttle SA in the 400 & 800 rpm range down to 8 deg. from 20.

 

Going from a 350 to a 383 is a 9% increase in displacement. In general the VE tables would need to be increased by that amount to make up for the lower cylinder volume value.

Then any thing that is done to increase the volumetric efficiency of the engine will also require an increase in the VE table values (cam, heads, exhaust, ...).

RBob.

 

That explains it.
Thanks for the help.